To avoid collision while driving, a vehicle cruise control system or a driver assistant system has been studied to put into commercial use. In such systems to avoid collision, automotive radar technologies play an important role that detects relative velocity of and distance to obstacles. In automotive radars, it is demanded that they can easily be mounted at low cost, and is also demanded that they apply systems capable of detecting, with sufficient resolution, distance to and velocity of obstacles.
As for automotive radar systems, such as pulse radar, pulse-compression radar (spread spectrum radar), frequency-modulated continuous wave radar (also referred as FMCW radar), and 2-frequency CW radar (CW: continuous wave), have been known. The pulse radar system transmits short pulses by which sufficient distance resolution can be obtained. The pulse-compression radar transmits a broadband signal with a broad pulse-width by spectral spread and obtains a specific distance resolution by compression (inverse spread spectrum). The pulse radar and the pulse-compression radar need a band of 150 MHz in order to obtain a distance resolution of 1 m and also need high-speed signal processing because of heavy correlational computation load, therefore those systems are disadvantageous to automotive radars that are required to be easily mounted at low cost.
Meanwhile, the FMCW radar system and the 2-frequency CW radar system can achieve a distance resolution of some 1 m with a receiver band of several dozen kHz, therefore they are expected to be widely applied hereafter to automotive radars. However, because continuous waves are used in those systems, reception signals include waves that have been reflected from moving objects and stationary ones at any distance away. Consequently, when a lot of heavy clutter is included, those systems have a problem in that they easily fail to estimate distance to and velocity of target objects.
In order to solve the problems above, a technology has been proposed, in which a beat signal during a frequency-ascending period and a frequency-descending period is spectrally analyzed in the FMCW radar systems, and when the frequency difference between a pair of spectrum peaks becomes a predetermined value, these spectrum peaks are determined as the waves reflected from stationary objects (clutter), so as to be removed (as described in Patent Document 1).
Also, as for a method of detecting distance to stationary objects and moving objects in the 2-frequency CW systems, a technology has been proposed, in which a slope period during which two frequencies vary is provided in a portion where the two frequencies alternate with each other (as described in Patent Document 2).
Patent Documentation 1                Japanese Patent No. 3102224        
Patent Documentation 2                Japanese Patent Laid-Open No. 2002-071793        